Initially and during the 1970's the major effort for most of the air bag research was in designing systems that would satisfy Motor Vehicle Safety Standard 208 (MVSS 208). Consequently, the auto companies concentrated their efforts on 30 mph frontal and angular barrier crashes. Only a small percentage of crashes staged by the auto companies were other than 30 mph barrier crashes and only a hand full were impacts other than barrier crashes. By contrast, for example, in real world experience the ratio of fatal pole crashes to fatal barrier type crashes is about 7:1.
The dominance of 30 mph barrier crash resulted in simplistic sensor criteria. In particular, it became universally accepted that if an occupant, represented mathematically as an unrestrained free mass, would strike the interior passenger compartment at 12 mph or above that an air bag was required. Since the maximum distance that an occupant can be positioned away from the passenger compartment interior is about two feet, it was computed that if the velocity change exceeded 12 mph prior to the time that the occupant displacement had exceeded 24 inches then an air bag was required. Once it was determined that an air bag was required for a given crash, then it was required that the sensor activate to initiate air bag inflation in time to permit 30 milliseconds of inflation time prior to the time that the occupant had moved 5 inches. This requirement always resulted in the air bag inflating in time for all types of accidents, and, in particular, for the 30 mph barrier crashes.
Initially and during the 1970's, in order to satisfy the sensor activation criteria for soft crashes such as angle barrier crashes and some car-to-car crashes, sensors had to be placed in the crush zone of the car. The crush zone is that portion of the car which experiences a velocity change early in the crash before the total car has slowed down. However, it was recognized in the mid-1970's that considerable economies in the air bag system could be realized through use of an all mechanical sensor in proximity to and integral with the gas generator. Through considerable research, development and evaluation, it was determined by the assignee Breed Corp. through computer mathematical modeling using data from hundreds of crashes supplied by seven of the worlds largest automobile manufacturers, that a sensor located out of the crush zone in the automobile can initiate air bag inflation in time to keep the injury level sustained by the driver below acceptable limits. The importance of this conclusion is that the cost of a driver-side-only air bag system, for example, would be reduced by about a factor of 5 and in addition there would be a dramatic improvement in system reliability.
A proposed driver's side air bag system is disclosed in the Bell U.S. Pat. No. 4,116,132. In this Bell Patent the sensor is placed on the side of the inflator housing. This known design has many disadvantages. The existence of a hole in the housing requires that the wall thickness be substantially increased to meet the pressure requirements, since it has been weakened by the presence of the hole. Notwithstanding the location of the sensor, the inflator housing must be designed to withstand several thousand psi of pressure caused by the burning of the propellant. Similarly this sensor itself in the Bell system, must have a housing which will withstand substantial pressures. Otherwise, the sensor will most likely rupture when the gas generator material is burning. If the sensor is outside the inflator, therefore, as shown in this Bell patent, the weight of the inflator will be significantly greater. This, in turn, increases the weight of the steering wheel and the bracketry which holds the steering column in position must be strengthened. This increase in bracket strength and bracket weight is extremely significant due to the fact that the extra inflator weight occurs at the end of a cantilevered shaft. This added weight also makes retrofitting vehicles more difficult due to this added bracketry requirement.
If the sensor is placed outside of the inflator and not physically attached to it, a serious decrease in system reliability results. The sensor now must be attached to the inflator either electrically, or with some sort of pyrotechnic transmission cord. Since there is a significant probability that this link could be broken, a system of monitoring the integrity of this link is probably required. This monitoring system would be undoubtedly electrical, requiring a connection to the vehicle battery. For the steering wheel mounted system, this would necessitate slip rings, a wire harness, an indicator light, and some electronic circuitry. This in turn results in a tremendous increase in system complexity, cost and added possibility of malfunction.
In addition to the many foregoing disadvantages the high cost of prior art air bag systems has been the primary impediment to their utilization.